This invention relates to a cooling system and a method for controlling a cooling system for an internal-combustion engine of a motor vehicle having a thermostatic valve which controls the quantity of coolant which flows through a short-circuit pipe and/or a coolant radiator from an engine outlet to an engine inlet, and which thermostatic valve can be switched by means of switching signals depending on operating parameters of the internal-combustion engine and/or environmental parameters from a higher control level of the coolant temperature to a lower control level of the coolant temperature, and having a fan which is assigned to the coolant radiator and can be switched by means of a fan control circuit.
A cooling system of the type generally described above is an object of German Patent Application DE 44 09 547.3 which is not a prior publication. In the case of this cooling system, a requirement-oriented cooling takes place, during which the cooling system is normally operated at a higher control level of the coolant temperature. In cases in which an increased demand for cooling output is expected and necessary, a switch-over of the thermostatic valve takes place to the lower control level of the coolant temperature. The fan, which in the case of this construction is assigned to the coolant radiator, is switched on by a fan control circuit when a switch-over takes place from the higher control level of the thermostatic valve to the lower control level. In this case, the switch-on duration of the fan is limited by a time function element.
A cooling system for an internal-combustion engine with a switchable thermostatic valve is also known (European Patent Document EP-B 0 128 365) which contains a separate fan operating control circuit for a fan assigned to a coolant radiator. The fan is operated as a function of the coolant temperature sensed by a temperature sensor with different output steps; that is, in the case of a lower coolant temperature, with a lower output and, in the case of a higher coolant temperature, with a higher output. When the thermostatic valve is switched over to a higher opening temperature, the switching-on of the fan is blocked at lower coolant temperatures in order not to counteract the reaching of the higher opening temperature.
It is an object of the invention to control, in the case of the cooling system of the initially mentioned type, the fan in such a manner that it is operated corresponding to the adjustment to the higher and the lower control level and promotes the reaching of the endeavored coolant temperature and of the respective demanded cooling output.
This object is achieved by providing a fan control circuit that contains a temperature comparison step which compares the ACTUAL temperature of the coolant with a desired value and, when the desired value is exceeded, generates a temperature signal which is applied to an input of an AND-element to whose other input the switching signal of the thermostatic valve is applied and which generates a switch-on signal for the fan.
In the case of this construction, the fan will only be switched on when the switching signal is present by means of which the thermostatic valve is switched back to the lower control level, and the given desired value is exceeded. Thus, the switching-on of the fan does not hinder the thermostatic valve from reaching the higher control level. Likewise, after the switch-over to the lower control level, this control level is reached faster because then the fan participates in the demanded, increased cooling output.
In one advantageous embodiment of the invention, the desired value for the temperature comparison step is in the range of the low control level of the thermostatic valve. Therefore, the fan participates in the cooling performance also in this control range in which an increased cooling output is required. In this case, the desired value advantageously corresponds approximately to the opening temperature of the thermostatic value adjusted to the lower control level. As a result, in the case of this setting of the thermostatic valve, the fan participates in the application and the control of the cooling output.
In another embodiment of the invention, the fan control contains a control step which determines the fan output as a function of the ACTUAL temperature of the coolant. Thus, the fan also acquires a control behavior by means of which it participates in the control of the coolant temperature. In an advantageous development, the control step contains a characteristic diagram in which coolant temperatures and assigned fan outputs are filed. This permits the control step to be adapted to the construction of the internal-combustion engine and/or the cooling system by means of a suitable characteristic diagram.
In a further development of the invention, the fan control circuit contains another temperature comparison step which compares the ACTUAL temperature of the coolant with a desired value and generates a temperature signal which represents an exceeding of the desired value and which is applied to a non-negated input of an (N)AND-element This (N)AND-element is an AND-element with one negated input and one non-negated input. The switching signal for the thermostatic value is applied to the negated input of the (N)AND element. The output of the (N)AND element generates another switch-on signal for the fan. By means of this embodiment, the fan is also switched on when an increased cooling output is required when a desired temperature level is reached, without any switch-over of the thermostatic valve to the lower control range. Since the fan participates in the cooling during the operation at the higher control level, advantages are obtained in the lay-out of the size of the coolant radiator.
In a further development of the invention, the desired value for the additional temperature comparison step is in the range of the higher control level of the thermostatic valve. It is advantageous for the desired value to correspond to the coolant temperature at which the thermostatic value has reached approximately 75% of its opening stroke.
In a further development of the invention, the fan control contains another control step which can be activated by the switch-on signal of the additional temperature step and which determines the fan output as a function of the ACTUAL temperature of the coolant. Also in the case of this further development, it is advantageous for the additional control step to contain a characteristic diagram in which coolant temperatures and assigned fan outputs are filed. It will then also be possible to adapt the control step to the construction of the internal-combustion engine and/or of the whole cooling system by determining a corresponding characteristic diagram.
In its basic construction, the cooling system may correspond to the cooling system illustrated and explained in German Patent Application DE 44 09 547.3. It contains a thermostatic valve which is equipped with a thermostatic working element. The thermostatic working element contains an expansion medium, particularly a wax mixture, which, in a predetermined temperature range, experiences a considerable change of volume and, in the process, moves out a working piston which opens up the thermostatic valve. The thermostatic valve controls the quantities of coolant which flow from an engine outlet of the internal-combustion engine through a coolant radiator and/or through a short-circuit pipe to the engine inlet of the internal-combustion engine. The lay-out of the expansion medium (wax mixture) determines the opening temperature of the thermostatic valve at which a flow through the coolant radiator starts and thus a control of the cooling, When the opening temperature is exceeded, the thermostatic valve carries out an opening stroke at which finally the flow cross-section for the coolant quantity flowing through the coolant radiator is opened up completely and the flow cross-section of the short-circuit pipe is closed completely.
The expansion medium is laid out in such a manner that the opening temperature is relatively high, for example, at 105.degree. C., while the full opening stroke is reached at approximately 120.degree.. As a result, an increased control level is obtained at which the coolant temperature is controlled to, for example, 115.degree. C.
When a higher cooling performance is required, as a function of operating parameters of the internal-combustion engine and/or environmental parameters, the thermostatic valve can be switched to a lower control level. For this purpose, the thermostatic working element is provided with an electric heating device by which the expansion medium can be heated to a temperature which is above the coolant temperature. As a result, the thermostatic valve opens up further so that a larger quantity of coolant flows through the radiator. Because of the increased cooling, the coolant temperature will then be lowered. As a result, the thermostatic valve will then operate at a lower control level or control range; that is, it opens at a coolant temperature of as low as 80.degree. C. so that then a coolant temperature of, for example, 85.degree. C. is set.
These and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.